Where time would normally be able to resist or absorb minor changes, the Third Doctor noted that some locations in space and time were temporal probability nexuses where multiple strands of causality were exposed and weak, and the smallest alteration could produce aberrant loops of existence or even new alternate timelines. (PROSE: The Eye of the Giant)

As related to cause and effect, i.e. causality, nexus points often served as areas for potential anomalies, since the Fourth Doctor stated that every point in time had its alternative. (TV: Pyramids of Mars)

In keeping with this, the "Wenley Moor Affair" was a crucial nexus-point in Earth's time stream because countless futures could diverge from that moment into alternate realities. (COMIC: Final Genesis)

Rose Tyler, having travelled to Donna's World, refused to reveal her name. She stated that "one word in the wrong place can change an entire causal nexus". (TV: Turn Left)

When asked why he couldn't just take the TARDIS back to the previous day, the Tenth Doctor recited, as if by rote, "I can't go back within my own timeline. I have to stay relative to the Master within the causal nexus." He earlier called the same set of events a convergence. (TV: The End of Time)

While discussing changing her personal past, Older Amy Pond capped a list comprised of destiny and causality with the nexus of time itself, a structure of causal nexus points that described or defined time. However, at the time she was referencing changing her own recent personal history from within a stable temporal anomaly that was generated artificially. (TV: The Girl Who Waited)

The Edifice's interior dimensions were mapped onto its exterior, making it the same size inside as outside, cancelling its dimensional transcendence. This also caused it to become a nexus point, affecting past and future events along the causal pathways, generating temporal anomalies in the resulting temporal pulses. (PROSE: The Ancestor Cell)